The partial oxidation of low value hydrocarbons such as alkanes and alkenes into high value products such as alcohols and epoxides is of great commercial interest. These oxidation products are not only valuable as is, but also as intermediates for specialty chemicals including pharmaceuticals and pesticides.
U.S. Pat. No. 4,410,501, issued Oct. 18, 1983 to Esposito et al., discloses a titanium-containing analogue of the all-silica ZSM-5 molecular sieve. This material (known as "TS-1") has been found to be useful in catalyzing a wide range of partial oxidation chemistries, for example the production of catechol and hydroquinone from phenol and hydrogen peroxide (H.sub.2 O.sub.2) and the manufacture of propylene oxide and cyclohexanone oxime from propylene and cyclohexanone, respectively. In addition, TS-1 can be used to catalyze the reaction of alkanes and aqueous H.sub.2 O.sub.2 to form alcohols and ketones. (See Huybrechts, D. R. C. et al., Nature 1990, 345, 240-242 and Tatsumi, T. et al., J.C.S. Chem. Commun. 1990, 476-477.)
TS-1 has many salient features, other than its catalytic abilities, which make it attractive as a commercial catalyst. Most importantly, it is a solid. This allows for easy separation from the reactants and products (typically liquids) by simple, inexpensive filtration. Moreover, this solid has high thermal stability and a very long lifetime.
Calcination in air at moderate temperatures (550.degree. C.) restores the material to its original catalytic ability. TS-1 performs best at mild temperatures (&lt;100.degree. C.) and pressures (1 atm). The oxidant used for reactions catalyzed by TS-1 is aqueous H.sub.2 O.sub.2, which is important because aqueous H.sub.2 O.sub.2 is relatively inexpensive and its by-product is water. Hence, the choice of oxidant is favorable from both a commercial and environmental point of view.
While a catalyst system based on TS-1 has many useful features, it has one serious drawback. The zeolite structure of TS-1 includes a regular system of pores which are formed by nearly circular rings of ten silicon atoms (called 10-membered rings, or simply "10 rings") creating pore diameters of approximately 5.5 .ANG.. This small size results in the exclusion of molecules larger than 5.5 .ANG.. Because the catalytically active sites are located within the pores of the zeolite, any exclusion of molecules from the pores results in poor catalytic activity.
U.S. Pat. No. 4,963,337, Oct. 16, 1990 to Zones, discloses the boron-containing zeolite designated therein "SSZ-33". This zeolite has some of its pores formed by 10-membered rings and others by 12-membered rings. This allows for larger molecules, which would be excluded from the pores of TS-1, to enter the pore system of SSZ-33.
It has now been discovered that when SSZ-33 is modified by replacing at least part of the framework boron atoms with titanium atoms, a catalyst results (designated herein as "Ti-SSZ-33") which has all the benefits of TS-1 without the drawback of its smaller pore system. In direct contrast to TS-1, in which the zeolite is synthesized in the presence of titanium, the aforementioned modification of SSZ-33 to Ti-SSZ-33 involves post-synthesis treatment of as-made samples of SSZ-33 to substitute titanium atoms for at least part of the boron atoms in the as-made SSZ-33.
A post-synthetic treatment of this type is described in Rigutto et al., Studies in Surface Science and Catalysis, 1994, 84c, 2245-2252 wherein the incorporation of titanium into boron-containing zeolite beta is disclosed. There, the post-synthesis treatment involved treatment of the boron-containing zeolite beta with titanium tetrachloride (TiCl.sub.4) followed by treatment with methanol.
While zeolite beta has a larger pore system than either TS-1 or SSZ-33, it is less thermally stable than those zeolites. Thus, there exists a need for a titanium-containing zeolite which has both relatively large pores and acceptable thermal stability. The zeolite of this invention, Ti-SSZ-33, satisfies that need.